A magnetic resonance imaging (MRI: Magnetic Resonance Imaging) apparatus is an apparatus for obtaining a tomographic image representing physical and chemical characteristics of a target body using a nuclear magnetic resonance phenomenon occurring when high frequency pulses are applied to the target body placed in a homogeneous static magnetic field, and particularly used as a medical use. The MRI apparatus mainly includes a magnet device for generating a homogeneous static magnetic field in an imaging region into which the object body is inserted, a gradient magnetic field coil device for generating a pulsed gradient magnetic field (gradient magnetic field distribution) of which intensity is specially inclined to apply positional information to the imaging region, an RF coil causing nuclear magnetic resonance phenomenon by radiating the high frequency pulses toward the object body, a receiving coil for receiving magnetic resonance signal from the object body, and a computer system for displaying a tomographic image through processing the received magnetic resonance signal.
Means for enhancing a performance of the MRI apparatus is provided by enhancing a static magnetic field intensity generated by the magnet device. The larger the static magnetic field intensity, the clearer and the more various tomographic images can be obtained, so that development is continued to have the higher static magnetic field intensity. Another means for enhancing a performance is provided by increasing the intensity of the gradient magnetic field and the gradient magnetic field pulses are driven at a high speed. These contribute toward shortening photographing time period and increase in an image quality and used by a high speed photographing method which has been used frequently in recent years. The high speed photographing method is enabled by a high speed switching and supply of a large current intensity, which are provided by an enhanced performance of a drive power source for the gradient magnetic field coil device.
Because pulsed currents flow in the gradient magnetic field coil device to generate pulsed gradient magnetic field, eddy currents are generated at a metal container part of the magnet device by the pulsed gradient magnetic field (residual magnetic field), so that the magnetic field by the eddy currents influence the tomographic image. Accordingly, the gradient magnetic field coil device includes the main coil for generating the gradient magnetic field in the imaging region, and a shield coil to prevent the pulsed gradient magnetic field (residual magnetic field) from leaking to an unnecessary part other than the imaging region. The shield coil reduces the residual magnetic field to suppress generation of eddy currents at the metal container part of the magnet device.
To suppress the generation of the eddy currents, it is necessary to manufacture the main coil and the shield coil in accordance with the intended design. In the actual manufacturing the main coil and the shield coil, spiral shapes are formed by cutting an electrical conductive material such as metal plates, in a saddle shape by a bending process. An insulation material of a resin is filled in a gap between multi-layers of the main and shield coils and hardened and fixed to each other. During a plurality of such manufacturing processes, a manufacturing error due to a slight difference from a configuration intended in designing may occur. When the manufacturing error is small, the residual magnetic field is low, so that the eddy currents generated at the metal container part are also low. Accordingly, eddy current magnetic field generated in the imaging region by the eddy currents is also low without influence on the tomographic image. However, when the manufacturing error becomes large, influence on the image by the eddy current becomes non-negligible.
Accordingly, to accurately manufacture the gradient magnetic field coil device, there is a proposed technology in which metal plates are shaped in a spiral shape by cutting process while the metal plates are being fixed on a pedestal having a curved plane (see, for example, Patent document 1). Further, there is another proposed technology in which generation of the eddy currents is reduced by increasing flow path resistances of the eddy currents by providing slits in the metal container part of the magnet apparatus.